A New Flavonoid Glycoside from Scutellaria barbata
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A NEW FLAVONOID GLYCOSIDE FROM Scutellaria barbata
Yang Liu, Xu-Hua Huang, Jia Chen, Jian-Hua Shao, and Chun-Chao Zhao*
A new flavonoid glycoside 1, together with nine known compounds, has been isolated from Scutellaria barbata D. Don. The structure of compound 1 was elucidated as acacetin 7-O-β-D-galactopyranosyl(1→2)-β-Dglucopyranoside by spectroscopic data (1D and 2D NMR methods) and chemical evidences. Additionally, compound 1 displayed potent inhibitory activity against α-glucosidase with an IC50 value of 8.9 μM in vitro. Keywords: Scutellaria barbata, Ban-Zhi-Lian, flavonoid glycoside, α-glucosidase inhibition. Scutellaria barbata D. Don, well known as Ban-Zhi-Lian, is a perennial herb natively distributed throughout China and Korea. The use of this herb in traditional Chinese and Korean medicine for treating tumors, hepatitis, cirrhosis, and other diseases has a long history. A large number of chemical constituents of S. barbata include alkaloids, flavonoids, steroids, and polysaccharides, among which flavonoids are the main bioactive component [1]. Previously, we reported a new phenolic glycoside from S. barbata [2]. In this paper, a new flavonoid glycoside with significant α-glucosidase inhibitory activity and nine known compounds were obtained from S. barbata. Compound 1 was obtained as a yellow amorphous powder with the molecular formula C28H32O15 as deduced by analysis of positive HR-ESI-MS m/z 631.1641 [M + Na]+ (calcd for C28H32O15Na, 631.1636). The positive Mg-HCl color reaction and 1D NMR analysis indicated its flavonoidic nature, and, in particular, 15 carbon atoms were ascribable to the aglycon and 12 to the sugar groups. In the 1H NMR spectrum of 1, the characteristic signals for the A, B, and C rings of the flavonoid [δ 6.45 (1H, d, J = 2.0 Hz, H-6) and 6.80 (1H, d, J = 2.0 Hz, H-8); 8.06 (2H, d, J = 8.2 Hz, H-2′, 6′) and 7.16 (2H, d, J = 8.2 Hz, H-3′, 5′); and 6.93 (1H, s, H-3)] together with the methoxyl proton at δ 3.88 (3H, s) were very similar to those of acacetin [3]. Acid hydrolysis of 1 yielded D-galactose and D-glucose, identified by Tanaka′s method [4]. The β-configurations of monosaccharide units were exhibited by the coupling constant values (J = 7.0 and 7.5 Hz) of two anomeric protons (δH 5.02 and 4.49). The HMBC (Fig. 1) cross peaks from H-1′′′ (δ 4.49) to C-2′′ (δ 81.6) established the 1→2 linkage between the galactose unit and the glucose moiety, and the correlation of H-1′′ (δ 5.02) to C-7 (δ 163.7) indicated that the sugar chain was connected to the C-7 of acacetin. Thus, compound 1 was determined as acacetin 7-O-β-D-galactopyranosyl(1→2)-β-Dglucopyranoside. The known compounds were identified as apigenin (2) [5], luteolin (3) [5], 3,5,7,4′-tetrahydroxyflavone (4) [5], 5,7,4′-trihydroxyflavanone (5) [6], 4-hydroxycinnamic acid (6) [7], 3,4-dihydroxycinnamic acid (7) [8], 3,4-dihydroxybenzoic acid (8) [9], 2-(3,4-dihydroxyphenyl)ethanol (9) [10], and 3-pyridinecarboxylic acid (10) [11]. Compound 1 showed noteworthy intestinal α-glucosidase inhibitory activity with IC50 = 8.9 μ
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